Both framerate and fun take a nosedive in the final part of the game.
Crysis starts off good enough with gameplay and jungle environment very much like its predecessor Far Cry. The levels aren't as big or open as those of Far Cry, but you'll still have plenty of options open to you. The graphics are clearly the best I've ever seen, provided you can turn everything to high or max detail settings. Unfortunately I think only an 8800 Ultra or better will do that for you at appreciable resolutions. I recently bought a brand new PC (8800GT, Q6600 cpu, 2Gb memory) and the first half of the game I would only get "smooth" frame rates at 1920x1200 at medium settings. Halfway through the game the changed environment made the FPS take a hit and forced me to go to 1440x900. Ugh. Well even at medium detail it probably looks better than anything else, but not by much.
The AI is somewhat of a mixed bag; they'll take cover and flank you, but when you turn on your cloak they seem to totally forget you're even there and tend to turn their back on the place where you just went invisible. Not good.
One of the selling points of the game is the highly destructive environment. Trees will snap like twigs all around in a fierce gun battle; it's a nice feature, but like all new technologies it's simply overused and overdone.
I already mentioned it; halfway through the game the s*** hits the fan. Too bad because from that point onwards the game is a 13 in a dozen slugfest, where it's just run & gun, and that's it. The aliens are boring to look at and not fun to fight. You're even thrown into a VTOL aircraft and there is no joystick support! To add insult to injury at some point you have to escape the pull of sucking air, which at the time just feels like the controls don't work.
Oh and can we have three hurrays for the latest trend in video gaming: .... cliffhanger endings. What we always wanted is finally reality. Gits.
Seriously they're going to have to get smart to prevent the expansion pack from being as boring as the last part of Crysis.
All in all, Crysis is not worth buying a new PC for and/or upgrading to Vista. Crysis is still a good game but it can't match the sheer greatness of its predecessor.